The present invention relates generally to surface modification of metallic materials and alloys resisting high temperatures. In particular, it relates to FeCrAl alloys that are modified by the application of a fluid, in particular a water-based silica dispersion.
Pure aluminum under normal atmospheric conditions forms a protective coating mainly consisting of Al-oxide on its surface, which makes it highly resistant to ordinary corrosion for a practically unlimited amount of time. Alloys containing a sufficiently high content of aluminum, such as FeCrAl alloys, also form aluminum oxide on the surface at exposure to high temperatures, e.g. at 1000xc2x0 C. However, such alloys can have a limited life especially when the alloy is in the form of thinner dimensions, such as 50 xcexcm thick foils. This is due to break-away oxidation, and oxidation of iron and chromium when the matrix is depleted of aluminum due to aluminum oxide formation. The most effective way to increase the life time, especially for foils, is to engineer the first-built, protective aluminum oxide layer.
Common, conventional methods for increasing the life of high temperature resistant alloys are:
a) Alloying with rare earth metals to decrease the growth rate of aluminum oxide; and
b) Introduction of a dispersion of small inclusions e.g. oxides, carbides or nitrides into the alloy.
At high temperatures, ferritic materials of the FeCrAl type have good oxidation properties but a relatively low strength. It is known that the strength at a high temperature, and in particular high temperature strength and creep strength, may be improved by adding materials that impede grain boundary sliding and dislocation movements in the alloy. Thus, grain boundary sliding may be counteracted on one hand by a reduction of the grain boundary surface, i.e., by increasing the grain size, and on the other hand by the introduction of stable particles that hinder the mobility of remaining grain surfaces, the order of magnitude of these introduced particles being 50 to 1000 nm. Moreover, the high temperature strength of the alloy may be increased by hindering dislocation movements. Particles for this purpose should preferably have an average size equal to or smaller than about 10 nm and be evenly distributed with an average distance between particles of 100 to 200 nm. These particles have to be extremely stable towards the metal matrix in order to avoid becoming dissolved or coarser with time. Suitable particle-forming materials to counteract grain boundary sliding and dislocation movements may be stable nitrides of primarily titanium, hafnium, zirkonium and vanadium, oxides of Al, Y, Th, Ca, . . . , carbides of Ti, Zr, V, Ta, Vd, . . . and mixtures of the above.
However, when making use of the above method, it has been established that the presence of Al, which is a relatively strong nitride former, leads to a decreased nitrogen solubility and makes the transport of nitrogen in the material more difficult. In turn, this brings about the inconvenience that a sufficiently fine separation of titanium nitride is not attained. Further, there is a risk that aluminum is bound in the form of aluminum nitride, which is detrimental for the oxidation properties of the alloy. This aluminum nitride may only be dissolved at high temperatures leading to the formation of titanium nitride. However, this results in too coarse a titanium nitride to satisfactorily counteract dislocation movements. Moreover, the presence of aluminum may also lead to separation of aluminum titanium nitride, which again is too coarse for the intended purposes.
Prior art citations which illustrate the nitride forming technique are EP-A225 047, EP-A-256 555, EP-A-161 756, EP-A-165 732, EP-A-363 047, GB-A-2 156 863, GB-A-2 048 955, EP-A-258 969, U.S. Pat. Nos. 3,847,682, 3,992,161, 5,073,409 and 5,114,470.
Thus, when applying nitriding methods to the above aluminum oxide forming high temperature alloys, the nitrogen will primarily be bound as aluminum nitride. This brings about two disadvantages. First, the ability of the alloys to form a protective aluminum oxide layer is limited. Second, the formed nitrides become too large and are not sufficiently stable.
In view of these inconveniences with nitrides, another method of improving the life of thin heat resistant materials is highly desirous, in particular for thin-walled articles. This method involves:
c) Increasing the aluminum content, or the contents of other elements with high oxygen affinity, in the matrix.
This may be achieved in different ways. According to one technique, gas atomization of aluminum metal is performed with a suitable inert gas, such as argon, and to which an alloy powder is introduced into the atomization gas. From the atomization process a mixture of aluminum powder and alloy powder is obtained. The amount of introduced alloy powder is adapted to the conditions of the aluminum flow, so that a desired aluminum content is obtained in the mixture. Thereafter the powder mixture may be encapsulated and compacted according to known methods. According to one known method, the powder mixture is filled into sheet-metal capsules, which are evacuated and sealed. A capsule filled with a mixture consisting of  greater than 3% by volume of aluminum powder, preferably between 8 and 18% by volume, and the rest alloy powder, may be isostatically cold-pressed to a relatively high density. Then the capsule is heated to a temperature near the melting point of aluminum. The solid or liquid Al phase then forms a solid solution together with the ferrite phase of the alloy.
Compacted capsules according to the above may then be heat treated to form, e.g., bars, wire, tubes and strip by a suitable method, such as extrusion, forging or rolling.
The alloy powder may also be mechanically mixed with an aluminum powder in such proportions that a desired final aluminum content is obtained. Thereafter, the mixed powder may be encapsulated and compacted according to the above description.
However, when using mixing methods, there is always a risk of demixing of the introduced components, leading to heterogenous alloys. Further, the processes may be costly and complicated, e.g. in view of the risks of the powder components being oxidized. Further, these methods often lead to production difficulties such as embrittlement during rolling.
Yet another technique for increasing the life of high temperature alloys is:
d) Cladding the material with aluminum foils, see for instance U.S. Pat. No. 5,366,139. According to this technique, one melts, moulds and rolls a ferritic stainless FeCr strip and cold-welds aluminum upon both sides at the end stage. By a heat treatment, the Al is dissolved into the FeCr strip and a FeCrAl composition is achieved. The advantage is that one avoids several of the difficulties with conventional production of FeCrAl. For example, the FeCrAl melts require more expensive linings in ovens and ladles. Further, it is more difficult to extrude the FeCrAl alloys and they are more brittle, which makes the handling of ingots and blanks more difficult and increases the risk of cracks during cold rolling.
Dipping of thin-walled details may also be done by the process as disclosed in U.S. Pat. No. 3,907,611, according to which a considerable improvement of the resistance against high temperature corrosion and oxidation of iron-based alloys is obtained. The method comprises an aluminization by dipping in melted aluminum, followed by heat treatments. The first heat treatment is performed to form an intermetallic surface layer and the second to obtain a good binding of it.
U.S. Pat. No. 4,079,157 discloses a method of fabrication of material suited for use in a thermal reactor according to which austenitic stainless steel is dipped in a bath of molten aluminum with silicon added thereto, and then receives heat treatment in specific temperature ranges, whereby preferential diffusion of silicon in the steel material is effected. The diffused silicon prevents diffusion of aluminum and ensures that thickness of plating layers remains at a value such that distortion of a plated element does not occur even after prolonged service. However, this method of aluminizing or hot dip galvanizing leads to thick coatings on the substrate, often 50 to 100 micrometers and is therefore to be regarded as a completely different approach.
However, these methods do not offer the satisfactory protection of thin-walled FeCrAl products against break-away oxidation.
JP-A-50-028 446 describes a method of washing a FeCrAl alloy with a suitable solvent or solution to remove halogens and then heat-treating to form a 40-100 xc3x85 thick Al2O3 film on the surface, to resist further oxidation. However, this document merely relates to the notoriously known fact that an alloy containing sufficient amounts of aluminum forms a protective oxide layer on its surface.
Therefore, it is an object of the present invention to provide a high temperature resistance alloy, and in particular a FeCrAl alloy, with a long useful life.
It is a further object of the present invention to make thin walled articles of FeCrAl alloys resistant at high temperatures during long times.
According to the present invention, this objects are achieved in a surprising way by modifying the surface of the alloy in accordance with the present invention.
According to a first aspect, the present invention provides a heat and oxidation resistant metallic material containing aluminum, the material comprising at least one of silicon and silicon-containing compounds applied onto its surface, the surface being in metallic or oxidized condition, thereby resulting in a surface layer or region containing amounts of the at least one silicon and silicon compounds and having an average thickness of 0.9 nm to 10 micrometers.
According to a second aspect, the present invention provides a method for surface modification of high temperature resistant metallic materials containing aluminum in order to increase oxidation resistance, the method comprising applying a fluid to said metallic surface.
For illustrative but non-limiting purposes, preferred embodiments of the invention will now be described with reference to the appended drawings. These are herewith briefly presented.